1. Field of the Invention
The present invention relates to a technique that generates driving waveforms to actuate driving elements of a print head.
2. Description of the Related Art
A color ink jet printer that ejects several color inks from a print head is one of the output devices of the computer. The ink jet printer expresses multiple tones by distribution of dots. In order to attain the smoother tone expression, some ink jet printers create variable size dots in respective pixels. Some ink jet printers carry out printing in both forward and backward passes of main scan, thereby enhancing printing speed.
The ink jet printer regulates the weight of ink droplets ejected from nozzles of the print head to create the respective dots. For example, the print head with piezoelectric elements regulates the size of each dot by controlling the meniscus or the shape of the ink surface at the nozzle opening and adjusting the ejection timing of the ink droplet. The driving waveform to drive the piezoelectric elements is varied to attain such control and adjustment.
FIG. 18 illustrates a prior art driving waveform to create the variable size dots. The driving waveform includes two element waveforms W1 and W2 that are output intervalically. The driving waveform has a interval T corresponding to one pixel division. The first element waveform W1 is used to create small size dots, and the second element waveform W2 is used to create medium size dots.
Large size dots are formed in response to both the first element waveform W1 and the second element waveform W2.
Various driving waveforms can be generated by a programmable signal generation circuit. The programmable signal generation circuit intervalically accumulates a preset value of voltage change in the driving waveform, that is, quantities of voltage change per unit time, with an adder, so as to determine the level of the driving waveform.
FIG. 19 is a block diagram illustrating the internal structure of a programmable signal generation circuit 100. FIG. 20 shows a process of generating a driving waveform by the prior art programmable generation. The driving waveform generation circuit 100 includes a memory 102, an accumulator 104, and a digital-to-analog (D-A) converter 106. Driving waveform data xcex94V1, xcex94V2, and xcex94V3, each representing a rate of voltage change in the driving waveform, are stored in the memory 102. Each of the driving waveform data corresponds to a quantity of voltage change in the driving waveform per interval t of a clock signal CLK. The driving waveform data xcex94V1, xcex94V2, and xcex94V3 read from the memory 102 are successively accumulated in synchronism with the clock signal CLK by the accumulator 104. The arithmetic operation gives 18-bit data as the result of accumulation. The upper 10 bits out of the 18-bit result of accumulation are subjected to the D-A conversion carried out by the D-A converter 106, so as to generate a driving waveform.
The number of reproducible tone levels increases as the number of available dot sizes and, in turn, as the number of element waveforms. There is, however, generally a restriction on the number of element waveforms included in one interval T. The prior art print head accordingly enables only a restricted number of different types of usable dots to be created.
The object of the present invention is to increase the number of different types of dots usable for printing.
At least part of the above and the other related objects is attained by a technique that selectively generates one of a plurality of different driving waveforms having different shapes at predetermined intervals of selection to actuate driving elements of a print head. The selective use of the plurality of different driving waveforms enables a variety of dots to be created without restriction on the number of element waveforms included in one interval.
In the technique of generating the driving waveforms in a programmable manner, changing the set of driving waveform data varies the resulting driving waveform. The switchover of the driving waveform is accordingly attained by appropriately rewriting the set of driving waveform data stored in the memory according to the desired driving waveform to be generated. This method, however, lengthens the time interval required for switching over the driving waveform. This is because no driving waveform is generated until the writing operation of a next set of driving waveform data is completed. The longer time interval required for the switchover may lower the printing speed and deteriorate the usability of the printing apparatus.
The technique of the present invention provides a specific number of memory areas, which are at least two memory areas and do not overlap one another at least partly, and changes over a working memory area used to generate the driving waveform, thereby switching over the resulting driving waveform. A set of driving waveform data used to generate each of the plurality of different driving waveforms is stored in each of the specific number of memory areas. This arrangement does not require the switchover operation of the working driving waveform to wait for the completion of the writing operation of the driving waveform data, thus attaining the high-speed switchover of the working driving waveform.
In the present invention, the predetermined interval of selection may corresponds to one pixel division. This arrangement effectively increases the number of different types of dots created in one pixel division during one pass of the main scan.
Here the term xe2x80x98one pixel divisionxe2x80x99 represents a time interval required for creating a dot at one pixel. The expression xe2x80x98one pixel divisionxe2x80x99 accordingly corresponds to the interval T of outputting the element waveforms W1 and W2 of the driving waveform shown in FIG. 18. The same principle is adopted in the case where the driving waveform includes three or more factor waveform element waveforms.
The term xe2x80x98one pixel divisionxe2x80x99 also has the following meaning. Printing is generally implemented by creating dots at respective pixel positions specified by a recording resolution. According to the relationship between the velocity of main scan and the driving speed of the print head, dots may be created on a raster line at a pitch of two or more pixels during one pass of the main scan. In one example, it is assumed that dots are created on a printing medium in a pattern shown in FIG. 1B in response to a driving waveform COM, which is output repeatedly at a interval T shown in FIG. 1A. The driving waveform COM output at the interval T enables dots to be created at every other pixel as shown in FIG. 1B. It is required to create dots in the residual alternate pixels by another pass of the main scan. In this case, the interval T still represents the time interval required for creating a dot at one pixel. The interval T accordingly corresponds to xe2x80x98one pixel divisionxe2x80x99.
The predetermined intervals of selection can be set at a variety of length, for example, intervals of different change rate of the driving waveform or intervals corresponding to one pass of the main scan of the print head.
The switchover of the driving waveform according to the present invention is implemented by a variety of embodiments discussed below.
A first embodiment provides a specific number of memory areas that corresponds to the number of different driving waveforms, and stores a set of driving waveform data corresponding to each of the plurality of different driving waveforms into each of the memory areas. In this arrangement, the sets of driving waveform data corresponding to the available driving waveforms are stored separately in the individual memory areas. This arrangement does not require the writing operation of the driving waveform data prior to the switch over of the working driving waveform, thereby having high speed of generating the working driving waveform.
In a second embodiment, a reading/writing operation of each memory area is done independently of operations of the other memory areas. The writing operation of one set of driving waveform data into one memory area is carried out in parallel with the reading operation of another set of driving waveform data from another memory area. This parallel operation is free from the time loss due to the writing operation of the driving waveform data.
In accordance with a concrete procedure of the second embodiment, while a first driving waveform is generated by utilizing a set of driving waveform data stored in a first memory area, another set of driving waveform data required for generating a subsequent second driving waveform is written into a second memory area. When the generation of the first driving waveform is completed, the working memory area is changed over from the first memory area to the second memory area. This arrangement enables the second driving waveform to be generated without any delay.
The expression xe2x80x98carrying out the writing operation in parallel with the reading operationxe2x80x99 is not restricted to the case of carrying out the writing operation simultaneously with the reading operation, but includes the case of changing over the reading operation and the writing operation to be carried out in a specified time interval.
In the second embodiment, two memory areas at the minimum are sufficient for the switchover of three or more different driving waveforms. This advantageously saves the memory capacity.
It is preferable that the respective memory areas are constructed by separate memory chips. This configuration readily attains the independent reading and writing operations for each memory area. When each memory chip has a terminal of a select signal that controls the reading and writing operations, it is desirable to input an inversion signal of the select signal, which is input into one memory chip, into the other memory chip. This arrangement effectively uses the single select signal to simultaneously control the reading operation from one memory and the writing operation into the other memory.
In the second embodiment, when a new set of driving waveform data to be written into the writing memory coincides with the existing set of driving waveform data already stored in the same writing memory, the writing operation may be omitted.
In accordance with one preferable application, the generation of the driving waveform according to the technique of the present invention may follow the steps of: selecting a driving waveform to be generated at the predetermined interval of selection; sequentially reading a set of driving waveform data, which corresponds to the selected driving waveform, out of the corresponding memory area at a preset timing; successively accumulating the read-out driving waveform data at a preset accumulation timing; and carrying out digital-to-analog conversion of the accumulation result, so as to generate a driving waveform signal.
The plurality of different driving waveforms having different shapes may include a reference driving waveform and a corrected driving waveform, which is obtained by correcting the reference driving waveform with a predetermined parameter that affect printing properties of the print head. In the print head that ejects ink for printing, the printing properties are synonymous with the ink ejection properties. The predetermined parameter may be at least one of an environmental temperature and an environmental humidity. The technique of the present invention switches over the working driving waveform at high speed and enables the effects of the predetermined parameter to be quickly reflected on the driving waveform, thus improving the accuracy of dot creation and the picture quality of the resulting printed image.
In the case where the driving waveform is corrected with the predetermined parameter, the arrangement of the second embodiment discussed above is preferably adopted; namely the reading operation from one memory is carried out in parallel with the writing operation into the other memory. The arrangement of the first embodiment requires all the sets of driving waveform data to be provided in advance corresponding to the possible variation of the parameter. This needs an extremely large memory capacity. Restriction of the memory capacity leads to restriction of the number of corrected driving waveforms, which undesirably lowers the accuracy of correction with the parameter. The second embodiment, on the other hand, carries out the writing and reading operations in parallel and thus effectively saves the required memory capacity for the corrected driving waveforms. This ensures the accurate correction with the parameter without undue restriction of the memory capacity.
The technique of the present invention is actualized by a variety of applications including a method of generating a driving waveform, a driving waveform generating apparatus, and a printing apparatus.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.